By a long sand trap an elongated shallow sand trap is understood in which the flowing-through essentially takes place along a straight horizontal path. During the flowing-through along said path the sand contained in or carried by the water has the opportunity to drop towards the bottom of the settling channel and to enter a collecting chamber formed there. Thereafter the sand is removed from this collecting chamber, e.g. by means of a transporting conveyor device which in many cases is constructed as a conveyor worm.
By sand a material is understood which has a particle size of &gt;0,2 mm in diameter. If possible the sand is to be separated without adhering organic constituents. Thus, the sand trap also serves the purpose of separating the sand from the sewage sludge. In so far it is important that the working direction of the transporting conveyor device is opposite to the direction of flow within the inflow channel. The long sand trap here in the area of the transport conveying device also serves the purpose for washing out the sand.
A long sand trap of the kind described above is known. It has a trapezoidal or roughly trapezoidal cross-section which is formed asymmetrically and has only one inclined wall. The transporting conveyor device is disposed within the collecting chamber at the lowest place of the settling channel, and thus, here, eccentrically with respect to the central longitudinal plane in the region of the continuous vertical wall of the channel. A classifying conveyor device which discharges the sand delivered by the transporting conveyor device in an upwardly inclined manner, belongs to this long sand trap. The classifying conveyor device must begin at a lower place than the transporting conveyor device in order to be in a position to receive and to discharge the sand delivered. It is known to arrange the classifying conveyor device, so to speak, adjacent the vertical wall and therewith outside the actual cross-section of the settling channel. This increases the cost of construction considerably. It is also known to dispose the transporting conveyor device eccentrically with respect to the central longitudinal plane of the cross-section of the settling channel, however, not at the lowest place, and to provide the classifying conveyor device at the lowest place, i.e. directly adjacent the vertical wall. In this case both the transporting conveyor device and the classifying conveyor device are disposed eccentrically with respect to the central longitudinal plane.
Finally, it is known to construct the cross-section of the settling channel symmetrically with respect to the central longitudinal plane and thus to form two inclined walls such that the lowest place and therewith the collecting chamber for the sand can be arranged in the central longitudinal plane of the settling channel. As a result of this arrangement the central longitudinal plane no longer is available for disposing the classifying conveyor device. Thus, the classifying conveyor device is provided eccentrically with respect to the central longitudinally plane.
These known long sand traps indeed are operated in advantageous manner such that the transporting conveyor device acts opposite to the direction of flow within the inflow channel, and therewith advantageously washes organic constituents out of the sand. However, the arrangements indicated above result in increased costs for the construction of the wall of the settling channel and the accommodation of the components. In all individual cases, the classifying conveyor device is provided eccentrically with respect to the central longitudinal plane such that the sand can be discharged only to one side of the classifying conveyor device, and namely to that side toward which also the classifying conveyor device is displaced eccentrically with respect to the central longitudinal plane. However, as it is required in sewage-treatment plants, depending on the structural conditions, to discharge the sand sometimes to the left and sometimes to the right of the classifying conveyor device, the necessity results to construct the long sand traps at one time as a left-hand discharge version and at another time as a right hand discharge version, whereby one version is a mirror image of the other version. Thereby the costs of production are increased.
From DE-PS 28 32 277 an arrangement for sewage-treatment is known which has also a long sand trap, wherein the transporting conveyor device for the sand in the collecting chamber as well as the classifying conveyor device for discharging the sand in an upwardly inclined manner are arranged in the central longitudinal plane of the channel cross-section. However, hereby the transporting conveyor device operates in the direction of flow, and the classifying conveyor device is seated downstream from the settling channel so that a considerable overall length is the result. Moreover, due to the operating direction of the transporting conveyor device, no sand is washed out, but the sand and the organic constituents are discharged together.
From DE-PS 30 28 541 a device is known for removing raked-off material, on the one hand, and sand, on the other hand, from inflow channels, particularly in sewage-treatment plants. A sand bucket wheel which has a comparatively largo diameter is fixed to the shaft of the rake. The sand bucket wheel takes over the sand from the transporting conveyor device and delivers the sand to a classifying conveyor device the axis of which is arranged in parallel to the axis of the rake. A common drive is provided for the rake, the sand bucket wheel, the transporting conveyor device and the classifying conveyor device. The additional sand bucket wheel with its large diameter increases the costs for the total installation considerably. Also the overall length of the device is comparatively large. The transporting conveyor device in the collecting chamber for the sand operates in a disadvantageous manner in the direction of flow.